Transpinnor-based transmission line transceivers and applications

ABSTRACT

Transceiver components for a transmission line are described. A driver includes a first network of thin-film elements exhibiting giant magnetoresistance, and a first input conductor inductively coupled to at least one of the thin-film elements in the first network. At least one dimension of each thin-film element in the first network is configured with reference to the characteristic impedance of the transmission line. A receiver includes a second network of thin-film elements exhibiting giant magnetoresistance, and a second input conductor inductively coupled to at least one of the thin-film elements in the second network. A termination impedance in series with the second input conductor has a value relating to the characteristic impedance of the transmission line.

RELATED APPLICATION DATA

[0001] The present application claims priority from U.S. Provisional Patent Application No. 60/372,712 for A TRANSPINNOR-BASED TRANSMISSION LINE TRANSCEIVER AND APPLICATIONS filed on Apr. 11, 2002 (Attorney Docket No. IMECP015P), the entire disclosure of which is incorporated herein by reference for all purposes.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to circuits and systems incorporating solid-state devices referred to herein as “transpinnors” and described in U.S. Pat. No. 5,929,636 and 6,031,273, the entire disclosures of which are incorporated herein by reference for all purposes. More specifically, the present application describes transpinnor-based transmitters and receivers for facilitating the reliable transmission of information via transmission lines and various applications thereof.

[0003] The vast majority of electronic circuits and systems manufactured and sold today are based on semiconductor technology developed over the last half century. Semiconductor processing techniques and techniques for manufacturing integrated circuits have become increasingly sophisticated resulting in ever smaller device size while increasing yield and reliability. However, the precision of such techniques appears to be approaching its limit, making it unlikely that systems manufactured according to such technique will be able to continue their historical adherence to Moore's Law which postulates a monotonic increase in available data processing power over time.

[0004] In addition, as the techniques for manufacturing semiconductor integrated circuits have increased in sophistication, so have they correspondingly increased in cost. For example, current state-of-the-art integrated circuits require a large number of processing steps to integrate semiconductor circuitry, metal layers, and embedded circuits, an issue which is exacerbated by the varied nature of the materials being integrated. And the demand for higher levels of complexity and integration continue to grow. The technical difficulties facing the semiconductor industry are well summarized by P. Packan in the Sep. 24, 1999, issue of Science magazine beginning at page 33, incorporated herein by reference in its entirety for all purposes.

[0005] Finally, there are some applications for which conventional semiconductor integrated circuit technology is simply not well suited. An example of such an application is spacecraft systems in which resistance to external radiation is extremely important. Electronic systems aboard spacecraft typically require elaborate shielding and safeguards to prevent loss of information and/or system failure due to exposure to any of the wide variety of forms of radiation commonly found outside earth's atmosphere. Not only are these measures costly in terms of dollars and weight, they are not always completely effective, an obvious drawback given the dangers of space travel.

[0006] In view of the foregoing, it is desirable to provide electronic systems which facilitate higher levels of integration, reduce manufacturing complexity, and provide a greater level of reliability in a wider variety of operating environments. As described in the aforementioned U.S. patents, such electronic systems are made possible by the advent of the all-metal, multi-purpose circuit element referred to as the “transpinnor.” It is therefore also desirable to facilitate the reliable communication of signals and data between the various components of such systems.

SUMMARY OF THE INVENTION

[0007] According to the present invention, electronic circuits and systems based on an all-metal solid-state device referred to herein as a “transpinnor” address the issues discussed above. More specifically, an embodiment of the present invention provides transmission line drivers and receivers implemented using transpinnor technology.

[0008] Thus, the invention provides a driver for a transmission line. The driver includes a network of thin-film elements exhibiting giant magnetoresistance, and an input conductor inductively coupled to at least one of the thin-film elements. The driver is operable to generate an output signal which is a function of a resistive imbalance among the thin-film elements and which is proportional to a power current in the network of thin-film elements. At least one dimension of each thin-film element is configured with reference to the characteristic impedance of the transmission line.

[0009] According to another embodiment, the invention provides a receiver for a transmission line. The receiver includes a network of thin-film elements exhibiting giant magnetoresistance, and an input conductor inductively coupled to at least one of the thin-film elements. The receiver is operable to generate an output signal which is a function of a resistive imbalance among the thin-film elements and which is proportional to a power current in the network of thin-film elements. The receiver further includes a termination impedance in series with the input conductor. The value of the termination impedance relates to the characteristic impedance of the transmission line.

[0010] A transceiver comprising the aforementioned driver and receiver is also described.

[0011] According to yet another embodiment, the invention provides a transceiver for a transmission line. The transceiver includes a driver comprising a first network of thin-film elements exhibiting giant magnetoresistance, and a first input conductor inductively coupled to at least one of the thin-film elements in the first network. At least one dimension of each thin-film element in the first network is configured with reference to the characteristic impedance of the transmission line. The transceiver also includes a receiver comprising a second network of thin-film elements exhibiting giant magnetoresistance, and a second input conductor inductively coupled to at least one of the thin-film elements in the second network. The receiver further includes a termination impedance in series with the second input conductor. The value of the termination impedance relates to the characteristic impedance of the transmission line.

[0012] A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1a shows a multilayer GMR film.

[0014]FIG. 1b shows a typical resistance curve for a GMR film such as the one shown in FIG. 1a.

[0015]FIG. 2a is a schematic diagram of a first transpinnor configuration.

[0016]FIG. 2b shows a plot of the output voltage of the transpinnor of FIG. 2a as a function of input current.

[0017]FIGS. 2c and 2 d show two alternative structures for the multilayer GMR film of FIG. 2a.

[0018]FIG. 3a is a schematic diagram of a second transpinnor configuration.

[0019]FIG. 3b shows a plot of the output voltage of the transpinnor of FIG. 3a as a function of input current.

[0020]FIG. 4 shows a transpinnor with a closed-flux configuration which is substantially the same schematically as the transpinnor of FIG. 3a.

[0021]FIG. 5 shows a transpinnor with an open-flux configuration which is substantially the same schematically as the transpinnor of FIG. 3a.

[0022]FIG. 6 illustrates the relationship between input current and output voltage for an all-metal GMR transpinnor.

[0023]FIG. 7 shows output voltage vs. input current for the GMR transpinnor of FIG. 6 with a small external bias applied.

[0024]FIG. 8 shows a multiple-input transpinnor configuration.

[0025]FIG. 9 is a circuit diagram of a transpinnor XOR gate.

[0026]FIGS. 10a and 10 b show transpinnors to operate as an AND gate and an OR gate, respectively.

[0027]FIG. 11 shows a transpinnor configured as a gated GMR differential amplifier.

[0028]FIGS. 12a and 12 b show a transpinnor configured as a switch according to a specific embodiment of the invention.

[0029]FIGS. 13a and 13 b show another transpinnor configured as a switch according to another specific embodiment of the invention.

[0030]FIGS. 14a and 14 b show a circuit symbol of a transpinnor switch and a circuit diagram of three transpinnor switches connected in series.

[0031]FIG. 15 is a simplified representation of a switching matrix.

[0032]FIGS. 16a and 16 b show 2 transpinnor-based switching matrices.

[0033]FIG. 17 is a simplified block diagram of a field programmable gate array.

[0034]FIG. 18 is a simplified block diagram of a field programmable system on a chip.

[0035]FIGS. 19A and 19B illustrate transmission line reflections for open and shorted terminations.

[0036]FIG. 20 is a representation of a transpinnor-based transmission line driver.

[0037]FIG. 21 includes representations of a plurality of transpinnor-based transmission line receivers.

[0038]FIG. 22 shows an exemplary switching characteristic of a static transpinnor receiver according to a specific embodiment of the invention.

[0039]FIG. 23 shows an exemplary switching characteristic of a pulsed transpinnor transceiver component with memory according to a specific embodiment of the invention.

[0040]FIG. 24 is a diagram illustrating the configuration of a pulsed transpinnor transceiver component according to a specific embodiment of the invention.

[0041]FIG. 25 is a timing diagram illustrating operation of the transceiver component of FIG. 24.

[0042]FIG. 26 is a simplified block diagram of an all-GMR implementation of a transmission line according to a specific embodiment of the invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

[0043] Reference will now be made in detail to specific embodiments of the invention including the best modes contemplated by the inventors for carrying out the invention. Examples of these specific embodiments are illustrated in the accompanying drawings. While the invention is described in conjunction with these specific embodiments, it will be understood that it is not intended to limit the invention to the described embodiments. On the contrary, it is intended to cover alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. The present invention may be practiced without some or all of these specific details. In addition, well known process operations have not been described in detail in order not to unnecessarily obscure the present invention.

[0044] “Giant magnetoresistance” (GMR) refers to the difference in the resistance that conduction electrons experience in passage through magnetic multilayer films which is dependent on the relative orientation of the magnetization in successive magnetic layers. For ferromagnetic materials, this difference occurs because the energy level for conducting electrons in a ferromagnetic layer is lower (by a few electron microvolts) for electrons with spin parallel to the magnetization rather than antiparallel. A GMR film is a composite structure comprising one or more multilayer periods, each period having at least two magnetic thin-film layers separated by a nonmagnetic conducting layer. A large change in resistance can occur in a GMR structure when the magnetizations in neighboring magnetic layers change between parallel and antiparallel alignments.

[0045] The property of giant magnetoresistance may be understood with reference to FIG. 1a which shows a multilayer GMR film 100 with a field coil 102 for supplying a magnetic field to GMR film 100. GMR film 100 contains magnetic layers of different coercivities separated by non-magnetic conducting layers (not shown). An ohmmeter 104 measures the resistance of GMR film 100 which changes as the input current I changes (see FIG. 1b); the dotted line represents the saturation of the high-coercivity film in the opposite direction to the solid line. As discussed above, if the magnetization direction of the magnetic layers of the first coercivity is parallel to the magnetization direction of the magnetic layers of the second coercivity, the resistance of the film is low. If the magnetization directions are antiparallel, the resistance is high.

[0046] GMR film 100 may be formed of one or more periods, each period having, for example, a cobalt layer characterized by a moderate coercivity, a copper layer, a permalloy layer characterized by a lower coercivity than the cobalt layer, and another copper layer. The different coercivities of the alternating magnetic layers make it possible to achieve an antiparallel orientation of the respective magnetization directions. The copper layers physically separate the magnetic layers, which otherwise would be tightly coupled by exchange forces. Consequently, it is possible to switch the magnetization in the low coercivity film without switching the magnetization in the high coercivity film. FIG. 1b shows a hypothetical resistance curve for an input current I which is not sufficient to reverse the polarity of the higher coercivity cobalt layer. As the current is increased, more of the low coercivity film switches, thus increasing the resistance. When the entire low coercivity film is switched there is no further change in resistance and the resistance curve levels off.

[0047]FIG. 2a shows a schematic diagram of a transpinnor 200 in which a GMR multilayer thin-film strip 202 is disposed in a bridge configuration with three resistive elements 204. A conductor 206 is wound around GMR film 202 for supplying a magnetic field thereto. An input signal is applied at terminals 208 and 210. Output terminals 212 and 214 give the output voltage, as indicated by a voltmeter. This configuration allows the output voltage to be zero as well as positive and negative. It should be noted that the output of transpinnor 200 may be represented by an ammeter rather than a voltmeter. That is, the transpinnor output may be either a voltage, i.e., operation in an “open-circuit” mode, or a current, i.e., operation in a “short-circuit” mode.

[0048] As is readily apparent, the input (between terminals 208 and 210) is completely isolated electrically from the output (between nodes 212 and 214) even for a DC input current I. The magnitude of the output is proportional to the applied B+ voltage and is limited only by the current carrying capacity of GMR film 202. FIG. 2b shows the output voltage of transpinnor 200 as a function of input current. If the values of resistors 204 are chosen correctly, the output voltage does not have a pedestal. That is, the curve crosses the y axis at y=0, and is not raised as in FIG. 1b. If the high coercivity film is reversed by either a strong input current or an external field, the polarity of the output is reversed, as shown by the dotted line in FIG. 2b. Again, it should be noted that a similar curve represents the situation in which the output of the transpinnor is a current rather than a voltage. A single-period GMR film 202 and a three-period GMR film 202 are shown in FIGS. 2c and 2 d, respectively, each having permalloy (216), cobalt (218) and copper (220) layers. The GMR films of FIGS. 2c and 2 d illustrate that various transpinnor configurations may employ single period and multi-period structures.

[0049] As mentioned, the output voltage or current of transpinnor 200 changes as the resistance of GMR film 202 changes and is proportional to the voltage drop across GMR film 202 as the current passes through it. The output voltage or current can be bipolar or unipolar, depending on the ratios of resistances chosen for the other legs (i.e., the bias can be positive, negative, or zero). Also, depending on the squareness of the B-H loop, the output voltage or current can either be linear or a threshold step function. In addition, if the GMR film 202 is constructed symmetrically about the center, the net magnetic field from the current passing through the film will be zero. Therefore, the only limits on magnitude of the current are the heating of GMR film 202 and/or electromigration. The GMR films may employ metals having high electromigration thresholds, such as copper, cobalt, nickel and iron.

[0050]FIG. 3a shows a schematic diagram of another transpinnor 300 having a different configuration. Instead of only one GMR film, transpinnor 300 employs four GMR films 302 arranged in a bridge configuration with conductor 304 wound through them for supplying a magnetic field thereto. As with transpinnor 200, the input of the device (between terminals 306 and 308) is isolated electrically from the output (between nodes 310 and 312) even with a DC input current. Also, the output voltage of transpinnor 300 is determined by the magnitude of B+ and the current carrying capacity of GMR films 302. As shown in FIG. 3b, transpinnor 300 has four times the output of transpinnor 200. Transpinnor 300 also has the advantage that the bridge is balanced to zero offset if all four films are identical.

[0051]FIG. 4 shows a transpinnor 400 with a closed-flux geometry which is substantially the same schematically as transpinnor 300. There is insulation (not shown) in the middle of transpinnor 400 where top GMR films 402 and 404 nearly touch bottom GMR films 406 and 408. The four GMR films form a Wheatstone bridge in which the resistance of each is variable. Input conductor 410 supplies the magnetic field and the output voltage is provided by output conductors 412 and 414. A bias voltage B+ is applied between nodes 416 and 418.

[0052]FIG. 5 shows a transpinnor 500 with an open-flux configuration which is substantially the same schematically as transpinnor 300. GMR film elements 502, 504, 506 and 508 form a Wheatstone bridge arrangement which requires only a single GMR deposition (i.e. the GMR layers are deposited in a single pump-down, with no patterning required between deposition of layers). Input conductor 510 was wound as a single layer of magnet wire. The closed-flux structure of FIG. 4 gives superior performance, especially for small-size devices, but involves multiple GMR depositions and patterning.

[0053]FIG. 6 illustrates the relationship between input current and output voltage for the all-metal GMR transpinnor shown in FIG. 5. The transpinnor was first initialized by saturating its four GMR film elements along the easy axes (i.e., parallel to the direction of film strips) with a magnet, and then applying input current until the magnetization direction of the permalloy layers in two of the elements switch completely (i.e., for maximum output from a Wheatstone bridge two resistors must be in the high resistance state and two in the low resistance state). After initialization in this manner, the data for the curve of FIG. 6 were taken. The solid curves, both positive and negative, were taken starting from the initialized state. The dashed curve is the remagnetization curve in which the applied field is made more negative (starting from the state of maximum output) in order to reestablish the initial magnetization state.

[0054] The solid curve of FIG. 6 shows a flat portion near the origin, then a rapid climb in output voltage when the input current reaches a threshold. It will be understood that this flat portion and threshold are desirable for digital applications, such as logic or selection matrices. The flat portion of the curve is largely due to the exchange bias between the permalloy and the cobalt layers. For linear applications, this portion of the curve can be removed either by the application of a small external bias, or by creating a symmetrical spin valve structure in which two cobalt layers are magnetized in opposite directions.

[0055]FIG. 7 shows an output voltage vs. input current curve for the GMR transpinnor of FIG. 6 but with a small external bias (e.g., 1.5 Oe) applied with a magnet in the easy direction (i.e., parallel to the film strips). As is evident, the exchange bias plateau around the origin has been essentially eliminated. As with FIG. 6, the solid lines begin with the initialized state, and the dashed line is the remagnetization curve. The finite hysteresis makes this transpinnor better suited for digital than for linear applications.

[0056] The GMR transpinnor of FIG. 6 has a rather large hysteresis in the permalloy of 1 Oe. However, permalloy coercivities of an order of magnitude smaller are found. This is of interest because the voltage and current gain of the GMR transpinnor are inversely proportional to the permalloy coercivity, and the power gain is inversely proportional to the square of the permalloy coercivity. The permalloy coercivity found in multi-period GMR films is routinely much lower than single-period GMR films. The reason is that the domain walls form in pairs in the closely spaced films of the multi-period devices, greatly reducing the magnetostatic energy of the walls. This is beneficial for linear applications because it increases the gain of the transpinnor. Unfortunately, a corresponding reduction in the coercivity of the cobalt layers is also found. This reduction is undesirable because at some point the magnetization direction of the cobalt layers begin to switch at a lower threshold than the magnetization direction of some of the permalloy layers. Obviously, the proper balance between these two parameters must be found for the particular application.

[0057] It is desirable in particular applications for the GMR transpinnor to have a gain greater than unity. The low-frequency gain of GMR transpinnors is a function of their fundamental parameters. Referring again to FIG. 5, input line 510 of transpinnor 500 is completely isolated from the output circuit. For the purpose of calculating the gain of transpinnor 500, let the input current be i, the input voltage be v, and the resistance of the input line be r. Furthermore, let the output voltage of transpinnor 500 be V, the resistance of the output circuit (including the GMR film) be R, and the current be I. Let us also introduce a variable to express the ratio of the percentage change in resistance caused by a small applied magnetic field. Where the shear is unimportant compared to the coercivity, this quantity, which we call the resistibility, X, is given by

X=GMR/(100H _(c))  (1)

[0058] where H_(c), represents the coercivity of the permalloy in the GMR film. The voltage gain of the GMR transpinnor of the present invention is proportional to the resistibility, and the power gain is proportional to the square of the resistibility.

[0059] The input line of the transpinnor produces a field. The ratio of field to the current by which it is produced is referred to herein as the coil efficiency, E. Generally speaking, the value of E increases dramatically as the size of the transpinnor decreases. If other parameters (including the resistance of the input line) stay the same, the voltage amplification is proportional to E, and the power amplification is proportional to the square of E.

[0060] Given the definitions of the various parameters of the transpinnor, the voltage amplification is given by

A _(voltage)=(R/r)IEX  (2)

[0061] and the power amplification is given by

A _(power)=(R/r)I ² E ² X ²  (3)

[0062] From (1) and (3) it becomes evident that the power amplification of transpinnor 500 is proportional to the square of the current, to the square of the GMR, to the square of the drive line efficiency, and inversely proportional to the square of the coercivity of the GMR film.

[0063] Some numerical examples of power amplification may be instructive. According to a first example, the input resistance is 0.8 Ohms, the resistance of the GMR film elements is 120 Ohms, the resistibility is 0.011/Oe, and the coil efficiency is 20 Oe/amp. If an input current of 500 mA is used, according to (3), the power amplification is 1.8. This is not a particularly good film.

[0064] According to a second example, the parameters are the same as for the first example above, except that the resistibility is 0.19/Oe. Now the power amplification is 541. This is higher than desirable for a logic tree, but may be reduced to a desirable value by appropriately decreasing the current.

[0065] According to a third example, a miniaturized transpinnor is configured as shown in FIG. 4, with the width of its features being on the order of one micron. The copper input conductor is 1 micron thick and 3 microns long. The input resistance is 0.05 ohms, the output resistance is 8 ohms, the coil efficiency is 6000 Oe/amp, the resistibility is 0.19/Oe, and the current is 1 mA. The power amplification is then 208.

[0066] The conclusion is that substantial power amplification can be achieved with GMR transpinnors using existing GMR film configurations. Additionally, amplification factors in the hundreds can be obtained regardless of whether the transpinnors are large or so small as to be at the limits of conventional lithography because the power amplification factor is independent of the size of the device. However, although GMR transpinnors scale so their power amplification doesn't degrade when the devices are miniaturized, the power handling capability of the devices diminishes, of course, as the device size diminishes. GMR transpinnors can be designed to give either high output current and low output voltage, or high output voltage and low output current. These parameters are determined by the aspect ratio of the GMR film. If the GMR film is a long narrow conductor, the output is high voltage and low current. If the GMR film is a short wide conductor, the output is low voltage and high current. The power amplification is relatively independent of the aspect ratio.

[0067] To get high power amplification, the following may be done:

[0068] (1) Make the input stripline as thick as possible in order to lower the resistance r. The power amplification depends only linearly on r, so this is less critical than the other steps.

[0069] (2) Make the resistibility as high as possible, either by raising the GMR or by lowering the coercivity of the permalloy.

[0070] (3) Make the GMR films as thick as possible to allow higher current without electromigration problems. This means many periods (e.g., 15 periods have been employed to obtain GMR of 15%).

[0071] Although low GMR films with very low coercivity can be used to construct GMR transpinnors with high power amplification, the resulting device may be inefficient. If overall power consumption is a consideration, one should use high GMR films. It is possible, for example, to make GMR films with GMR of more than 22%.

[0072] There are a wide range of applications for which the transpinnor represents a significant advance. For example, transpinnors may be employed to implement nonvolatile logic gates, i.e., gates which maintain their states when power is removed. Additionally, because all-metal films exhibit much greater resistance to damage by radiation than semiconductors, transpinnors may be employed to implement intrinsically radiation-hard electronics.

[0073] The curve shown in FIG. 7 exhibits hysteresis. Although this is not harmful (and may indeed be useful) for logic devices, for linear transpinnor performance, the hysteresis loop needs to be closed and straightened in a finite operating region; additionally, films with very low-coercivity should be used. In general, the shape of the hysteresis loop of thin films depends on the direction of the applied fields. Different approaches to achieve anhysteretic GMR films for transpinnor operation in the linear region based on three methods of eliminating hysteresis and distortion from GMR films are described. One approach is the application of a transverse (i.e., perpendicular to the easy direction) bias field having a magnitude slightly larger than the anisotropy field of the low coercivity element; the signal to be amplified is applied as a varying easy-axis magnetic field. This bias field can be supplied by an external coil or magnet, by individually deposited magnets on each amplifier, or by a current in a stripline. The effect of the bias is to eliminate the hysteresis and to greatly increase the longitudinal permeability, as described in two publications, Longitudinal Permeability in Thin Permalloy Films, E. J. Torok and R. A. White, Journal of Applied Physics, 34, No.4, (Part 2) pp. 1064-1066, April 1963, and Measurement of the Easy-Axis and H _(k) Probability Density Functions for Thin Ferromagnetic Films Using the Longitudinal Permeability Hysteresis Loop, E. J. Torok et al., Journal of Applied Physics, 33, No. 10, pp. 3037-3041, October, 1962, the entire disclosures of which are incorporated herein by reference for all purposes. The mathematics in these publications can be used to show that when a GMR film of resistance R, having one or more low coercivity layers (e.g. permalloy) with anisotropy field H_(k), is biased with a hard axis field H_(t)>H_(k), and to which a small easy axis field dH_(L) is applied, the film will have a corresponding resistance change, dR, given by

dR/dH _(L)=(GMR)R/(H _(T) −H _(k))  (4)

[0074] where GMR is the maximum resistance change, and H_(T) must be larger than the maximum H_(k) of any region of the film. This differential resistance change can be quite large if the inhomogeneity of the film is small, and the corresponding amplification can be large. This is a sensitive method of achieving anhysteretic GMR films by a transverse-biased permeability. It results in an analog signal with a linear response within a certain range.

[0075] In another approach to eliminating the hysteresis, the permalloy layer in the transpinnor is driven and sensed in the hard direction. The cobalt layer is deposited so that its easy axis is parallel to the hard axis of the permalloy. this is accomplished by saturating the cobalt layer during its deposition at 90 degrees from the easy axis of the permalloy. This method does not generally require a bias field during operation; the exchange bias between the high coercivity layer(s) and the permalloy layer is normally sufficient to prevent the hard-axis loop from opening. The sensitivity of the hard-axis-driven film is not as good as in the approach based on the transverse-biased permeability (described above), but the linearity extends over a broader range and this method is easier to implement in that it avoids biasing in the hard direction and driving in the easy direction.

[0076] Yet another approach involves a sampling method. A pulse is applied to the transpinnor between each data sample. The pulse is of sufficient amplitude to saturate the permalloy layers in the transpinnor to an initial state that is the same regardless of whatever signal was applied in between. The frequency of the applied pulse should be higher than the highest frequency of interest in the signal to be amplified. The result of using narrow pulses to reinitialize the magnetic material before each data sample is to erase the magnetic history and to eliminate the hysteresis in the output. The output can be sensed either with sampling techniques or as an analog output with a low-pass filter.

[0077] It is generally understood that all possible electronic circuits, analog and digital, can be implemented using active components, e.g., transistors, in combination with four basic passive components, i.e., resistors, capacitors, inductors and transformers. It is also well known that neither inductors nor transformers are available in semiconductor bipolar technology. By contrast, the GMR transpinnors can be employed to provide both of these components. In fact, they are well suited to provide the basis of a variety of analog, digital and mixed general-purpose all-metal circuits, subsystems and systems. Since capacitance and resistance can be implemented with the same metal technology as that used for the passive transformer and the transpinnor, all these components can be combined very effectively on the same substrate to produce a comprehensive variety of all-metal circuits. Unlike semiconductor chips, whose performance suffers below a critical size, the characteristics of GMR devices improve as the dimensions are decreased.

[0078] Biased in the appropriate operating region, GMR transpinnors can be used as basic building blocks of logic gates, thereby providing the foundation for GMR-based digital electronics. While logic elements can be made with combinations of transpinnors, just as with transistors, there is another alternative. Various logic operations can be implemented with a single transpinnor. These transpinnors have more than one input line. Examples of such transpinnors are shown in FIGS. 8 and 9.

[0079]FIG. 8 shows one such all-metal GMR transpinnor 800 and two drive lines. Four GMR films 802 are tied together in a folded Wheatstone bridge configuration. Each GMR film 802 is shown as a rectangular strip with its easy axis oriented in the long direction. Flux closure is also along the easy axis, but is not shown. The two drive lines (gates #1 and #2) are deposited conductor strips. The application of current on gate #2 tends to magnetize all four GMR films in the same direction. The application of current on gate #1 tends to magnetize adjacent GMR films oppositely. With the proper pulse combinations one can use half-select pulses to magnetize the high-coercivity layers positively or negatively in one direction, or to magnetize alternate strips in alternate directions.

[0080] As mentioned above, when a transpinnor is balanced, its output is zero. An input current which exceeds the threshold for switching a lower-coercivity layer in one or more of the GMR films can change the film resistance, thus unbalancing the transpinnor, resulting in an output signal. Particular types of logic gates can be realized from the basic transpinnor by specific configurations of input lines and by suitable choices of input current values. Additional characteristics affecting the operation of transpinnor logic gates include the choice of resistors through which a given input current passes, the current polarities in selected resistors, and the direction of the magnetic field produced by the input current relative to the magnetization of the lower-coercivity layers in the transpinnor.

[0081] Two procedures are useful in implementing logic gates with a single transpinnor. One involves setting the transpinnor threshold which is determined by the coercivity of the low-coercivity layers in the GMR film. Various ways of establishing the coercivity of a thin film are known in the art. Thus, the threshold is set by choosing or adjusting the coercivity of at least one of the low-coercivity layers in the GMR films of the transpinnor. The other procedure involves switching the polarity of the GMR films which is determined by the magnetization orientation of all the film layers. The polarity of the transpinnor is thus switched by reversing the direction of magnetization of all layers of all GMR films in the transpinnor.

[0082] According to various embodiments, the balancing of transpinnor GMR elements is accomplished using a technique known as magnetoresistive trimming in which the magnetization of selected GMR elements are manipulated to achieve the desired balance. Magnetoresistive trimming techniques are described in International Publication No. WO 02/05470 A2 entitled MAGNETORESISTIVE TRIMMING OF GMR CIRCUITS published Jan. 17, 2002, the entire disclosure of which is incorporated herein by reference for all purposes.

[0083] Logic operations which can be implemented with a single transpinnor include the following:

[0084] AND gate: A transpinnor will not switch unless the sum of fields from the input lines exceeds the switching threshold. An AND gate is defined as one that yields no output unless all of its inputs are logical “1”s. If the transpinnor has n input lines, and the amplitude of each input pulse is (1/n)^(th) of the threshold, then the transpinnor is an AND gate.

[0085] NAND gate: This is the inverse of the AND gate and gives an output if and only if all inputs are zero. A transpinnor NAND gate is made similarly as the AND gate, by reversing the magnetization of all elements so that the gate will just switch if all n inputs are logical “0”s and not switch if one or more are a logical “1”.

[0086] OR gate: The definition of an OR gate is one that gives an output if one or both inputs are a “1”. This can be made by setting the threshold of a transpinnor such that a single input is sufficient to switch the film.

[0087] A practical problem is presented by the fact that different switching thresholds are required for different single transpinnor logic gates. There are, however, a variety of ways in which these thresholds may be adjusted for different types of gates on the same substrate. These include manipulation of the order of deposition because the order strongly influences the coercivity of both the low and high coercivity films. This method involves additional deposition steps. Another method of adjusting the switching threshold for a particular transpinnor is derived from the fact that the magnetic field from a current carrying stripline depends on the width of the strip line.

[0088] NOR gate: The definition of a NOR gate is one that gives an output if one or both inputs are a “0”. This is merely the inverse of an OR. This can be done by reversing the polarity of the GMR films as in the above case of a NAND.

[0089] NOT gate: A NOT gate is an inverter that changes the polarity of an input pulse from positive to negative and vice versa. This is easily done with a transpinnor by reversing the polarity of the input winding, or by interchanging the power terminals.

[0090] Exclusive OR (XOR) gate: This is a gate that gives an output if one and only one of the inputs is a “1”. This can be done with a transpinnor such that one input is sufficient to switch the low-coercivity element, yielding an output, while two or more pulse inputs yield a field large enough to switch the high-coercivity element as well, yielding zero output. The gate must be reset after each use.

[0091] A circuit diagram of a transpinnor-based XOR gate 900 is shown in FIG. 9. As shown, input current 1 goes through resistors R1 and R3 and input current 2 goes through resistors R2 and R4. If the currents in both inputs are less than the switching threshold, the output is zero. If the current in one and only one of the two input currents is above this threshold, then the resistance of either pair of resistors changes, the transpinnor becomes unbalanced, and an output signal is generated. If both input currents are above the switching threshold, all four resistors change equally (if properly trimmed), the transpinnor remains balanced, and the output signal is zero.

[0092] A circuit diagrams for other transpinnor configurations are shown in FIGS. 10a and 10 b which, according to various embodiments, are used to implement AND and OR gates. Unlike XOR gate 900 in which one input goes through GMR elements R1 and R3 where the other goes through R2 and R4, both inputs for AND gate 1000 and OR gate 1050 go through all four elements. Referring now to FIG. 10a, AND gate 1000 is configured to function as an AND gate by selecting the current polarities such that the current from input 1 runs opposite to the current in input 2 through R2 and R4, and in the same direction through R1 and R3. If the currents in both inputs are less than half the switching currents, all four GMR elements remain unchanged, the transpinnor remains balanced, and the output of gate 1000 is zero.

[0093] If the current in one, and only one, input is above the switching threshold, all four GMR elements change equally, the transpinnor remains balanced, and the output of gate 1000 is zero. If, on the other hand, the currents in both inputs are above the switching threshold (and thus the net current through R2 and R4 is below the switching threshold), the transpinnor becomes unbalanced and gate 1000 produces an output signal.

[0094] Referring now to FIG. 10b, operation of gate 1050 as an OR gate is achieved because the input lines generate magnetic fields in GMR elements R1 and R3 opposing the directions of the magnetization vectors in the lower-coercivity layers of these elements, and magnetic fields in R2 and R4 in the same directions as the magnetization vectors in the lower-coercivity layers of these elements. With such a configuration, a sufficiently large current through R1 and R3 will change their resistances but not the resistances of R2 and R4, unbalancing the transpinnor and thereby producing an output.

[0095] If currents in both inputs are less than half the switching current, all four GMR elements remain unchanged, the transpinnor remains balanced, and the output of OR gate 1050 is zero. However, if the current in either or both of the inputs are above the switching current, the resistances of elements R1 and R3 change while those of R2 and R4 remain the same, the transpinnor becomes unbalanced, and OR gate 1050 generates an output signal. It will be understood that the net current through R2 and R4 should not be sufficient to produce a magnetic field which could switch the lower-coercivity layers of these elements.

[0096] For digital applications, transpinnors with sharp thresholds and square-pulse outputs are desirable. For analog applications, a linear response is better. Transpinnors operating in the linear region can be used to develop a full complement of basic analog circuits, sufficient to create general-purpose analog circuitry based on GMR films.

[0097] A specific example of a transpinnor operating in the linear region for application to signal amplification illustrates some of the unique advantages of the dual functionality of the transpinnor over silicon technology. Differential amplifiers are typically used to eliminate common-mode signal and common-mode noise within the frequency range of their operation. As discussed above, the range of operation of the transpinnor in its transformer function extends from (and including) dc to the high-frequency cutoff limit. The GMR transpinnor can advantageously be utilized in its transformer function to remove common-mode signal in the differential-input mode, as well as in its transistor function to amplify a low signal in the single-ended output mode. In low-signal amplification, GMR transpinnors have the additional advantage of eliminating the problem of offset voltage at the input that is so troublesome in silicon integrated circuits. It should be noted that a high premium is paid in silicon technology to achieve low-offset input voltage for integrated differential amplifiers. That is, low-offset input voltage is achieved in silicon circuits only at the expense of degrading other parameters. No such price is associated with the use of transpinnors because of their dual transformer/transistor properties. Specifically, the input signal is applied to a differential input having the properties of a transformer primary with an additional advantage of flat low-frequency response inclusive to dc. The output signal is amplified by an output having transistor properties. Transpinnors are thus especially well suited as differential amplifiers.

[0098]FIG. 11 shows a gated GMR differential amplifier 1100. Once again, four GMR films 1102 are arranged in a Wheatstone bridge configuration. Two input lines 1104 and 1106 supply a switching field to the permalloy layers in GMR films 1102. If the signals on lines 1104 and 1106 are identical, no switching takes place and the output (between nodes 1108 and 1110) is zero. Any common mode noise is thus rejected. All four lines (gate lines #1 and #2 and input lines 1104 and 1106) are electrically isolated, i.e., there is no electrical connection between them or to GMR films 1102 in gated differential amplifier 1100.

[0099] Since transpinnors are current driven devices, an important parameter is the output current of a given transpinnor for a given input current. This determines whether one transpinnor can switch another, for example, or how much amplification can be achieved. Of particular interest is the dependence of the amplification factor A=i_(out)/i_(in) on the power supply to the transpinnor and on its parameters. This relationship is given by:

A=π1000gmr VL/(H _(c) w ² R _(sq))  (5)

[0100] where V is the power supply voltage in volts, gmr is the fractional GMR value of the film (i.e., the GMR value is normally quoted as a percentage), H_(c) is the coercivity in Oe, w and L are the GMR strip width and length in microns, and Rsq=r/(L/w) is the sheet resistivity in ohms per square of a GMR film with resistance r (ohms per square is a standard term in thin film technology because the resistance from edge to edge of a thin film square is independent of the size of the square).

[0101] The field H produced by i_(in) in a stripline of width w is given by:

H=2πi _(in) /w  (6)

[0102] and i_(out) is given by:

i _(out)=10³ gmr v/(2r)  (7)

[0103] where H is in Oe, i_(in) and i_(out) are in mA, w is in microns, and V is in volts.

[0104] Many transpinnor-based devices require one transpinnor to switch another transpinnor. Examples include a transpinnor shift register, a transpinnor selection matrix, and a transpinnor multistage amplifier. When a transpinnor is used to switch another transpinnor, the output current of the switching transpinnor becomes the input current of the transpinnor to be switched. A single transpinnor can readily switch multiple transpinnors as shown by the following numerical examples of the performance characteristics of several transpinnor-based devices:

[0105] 1) shift register: In a transpinnor shift register, one transpinnor switches an identical transpinnor which, in turn, switches another identical transpinnor, an so on. An amplification factor of 1 is required. For w=L=5 microns, H_(c)=1 Oe, gmr=0.06, and R_(sq)=6 ohms per square, a power supply voltage of 0.168 is required (see equation (5)).

[0106] 2) amplifier: For a power supply voltage of 3 volts on a chip, with the other parameters the same as for example 1 above, the amplification factor is 18.

[0107] 3) branching logic: For the same parameters as in example 2, one transpinnor can switch a total of 18 other transpinnors.

[0108] 4) smaller transpinnors: If, from the examples above, w and L were both reduced by a factor of 5 to 1 micron, the required voltage for an amplification factor of 1 would also be reduced by a factor of 5 to 33.6 mV. Thus, for a 3 volt supply, an amplification factor of 90 can be achieved.

[0109] 5) different aspect ratios: For L=5 microns (as in example 1) and w=1 micron, the required voltage for an amplification factor of 1 is reduced to 6.7 mV.

[0110] 6) single-transpinnor comparator design: a comparator is a high gain differential amplifier, easily saturated, e.g., FIG. 11; For L=10 microns, w=1 micron, V=0.2 volt, Hc=1 Oe, gmr=0.06, and Rsq=6 ohms/sq for the GMR films, the amplification factor is 63 according to equation (5), and the output current of the comparator is 0.1 mA according to equation (7). For decoder logic with w=0.5 micron, the magnetic field applied to the decoder logic is 1.26 Oe according to equation (6), large enough to drive the decoder logic.

[0111] 7) comparator power dissipation: For the same parameters as in example 6, the resistance of each GMR element of the transpinnor is 60 ohms. This is the effective resistance between the power supply and ground of the transpinnor. For a supply voltage of 0.2 volt, the power dissipation of the comparator is {(0.2 volts)²/60 ohms}=0.67 mW.

[0112] The foregoing examples illustrate that even transpinnors with modest GMR values can achieve enough gain to perform the analog and logic functions required to implement a wide variety of circuits including, for example, a field programmable gate array (FPGA) and a field programmable system-on-a-chip (FPSOC) as will be described below.

[0113] According to various embodiments of the invention, transpinnors may be configured to operate as switches that are nonvolatile like EEPROMs yet are characterized by the programming speed associated with SRAMs. FIGS. 12a and 12 b show exploded views of a transpinnor switch 1200 designed according to one such embodiment. The nonmagnetic conductor layer in each GMR film separating the high coercivity layer 1208 (e.g., cobalt) and the low coercivity layer 1206 (e.g., permalloy) is not shown. The basic operation of switch 1200 involves the selective application of a large enough switching current on switch conductor 1204 to set the directions of the magnetization vectors of both the higher and lower-coercivity layers of GMR elements R1 and R4, while the magnitude of the input current on input conductor 1202 is only large enough to switch the lower-coercivity layers of R1 and R3.

[0114] As described above and generally speaking, transpinnor technology operates by impressing a specific magnetization direction on a lower-coercivity layer (e.g., permalloy). In digital applications, two opposing magnetizations of this layer correspond to two logic levels. By contrast, the higher-coercivity layer (e.g., cobalt) in such applications remains pinned to a particular magnetization.

[0115] As will be discussed, reconfigurability of a programmable SOC may be achieved through a transpinnor switch which includes two nonmagnetic, conductor layers inductively coupled to various ones of the GMR films of which the transpinnor is composed, e.g., FIG. 12. In general and as discussed above, the current in the input conductor of the transpinnor switch is large enough to set (reverse) the magnetization of the lower-coercivity layers of the GMR films to which the input conductor is coupled but not that of the higher-coercivity layers. By contrast, the current in the switch conductor can be large enough to set (reverse) the magnetization of both layers in the GMR films to which the switch conductor is coupled.

[0116] The basic idea of the transpinnor switch in digital applications is to use the switch current to control the magnetization of the higher-coercivity layers in selected GMR films of the transpinnor in such a way that the output is either a logic signal following the input, or zero irrespective of the input. It should be noted that there are various ways of configuring the transpinnor switch to realize this functionality. That is, non-magnetic conductors may be inductively coupled to various subsets of the GMR films to achieve this functionality and remain within the scope of the invention.

[0117] According to one embodiment shown in FIGS. 12a and 12 b, each of the input conductor 1202 and the switch conductor 1204 are inductively coupled to two of the GMR films of the transpinnor 1200. That is, input conductor 1202 is coupled to GMR films R1 and R3, and switch conductor 1204 is coupled to R1 and R4. The nonmagnetic conductor layer between the cobalt and permalloy in each GMR film is not shown for simplicity.

[0118] Transpinnor switch 1200 is initialized with all magnetizations in all four films parallel to one another (not shown). In this initialized state the resistances in all four films are low and the transpinnor is balanced, so there is no output when a power voltage is applied. A current is then applied via input conductor 1202 to reverse the magnetization in the low coercivity layers 1206 in films R1 and R3, resulting in a magnetization which is antiparallel to that of high coercivity layers 1208 as shown in FIG. 12a. This increases the resistance of films R1 and R3, unbalances the bridge, and produces a current in the output terminals when power is applied. Thus, in this configuration switch 1200 is “on,” i.e., an asserted logic signal on input conductor 1202 produces corresponding logic signal on the output.

[0119] To turn switch 1200 “off,” a switch current is applied via switch conductor 1204 to set the magnetizations in the high and low coercivity layers of R1 and R4 in a direction opposite the previous magnetization state of the high coercivity layers. Then, a small current is applied via switch conductor 1204 to reverse the low coercivity (but not the high coercivity) magnetizations in R1 and R4. Finally, a current with the same polarity as that used to orient the low coercivity layers in R1 and R3 in FIG. 12a is applied via input conductor 1202. The result is that R1 and R2 are low, R3 and R4 are high, as shown in FIG. 12b, and the transpinnor switch is balanced. Now, when an input current of the same polarity as prior to the reversal of the high coercivity layers is applied, the bridge remains balanced and no output is generated, i.e., the switch is “off,” irrespective of the asserted input logic level.

[0120] According to various embodiments, several transpinnor switches designed according to the invention may be connected in series to route a single input signal to a variety of circuits within a system. For example, a two-switch device may be configured with one switch “on” and the other “off” such that one of two circuits in a programmable system is enabled while another is disabled. As described above, the low coercivity layers in the “off” switch do not function, i.e., the output current is zero irrespective of which of the two logic levels is asserted at the input, while those in the “on” switch do, i.e., the output signal corresponds to the input signal. However, when the high coercivity layers of the appropriate films in both switches are magnetized in the other direction, the roles of the low coercivity layers in the two switches are reversed. Reconfiguration is thus achieved by simultaneously turning the “on” switch “off” and vice versa.

[0121] It should be noted that, according to some embodiments, the transition between balanced and unbalanced transpinnor configurations—and hence between output and no output—can be realized by reversing the polarity of the input signal to set the magnetization of the soft layer alone. However, this would obviate the use of a signal of given polarity to operate the logic circuitry in a given configuration.

[0122] According to various embodiment of the invention, there are a variety of ways in which transpinnor switch 1200 may be reconfigured. For example, we have just discussed shutting off the switch by reversing the magnetization of the high coercivity layers in R1 and R4. This shuts off the switch regardless of the input to the low coercivity layers of the films with which the input conductor is associated. It should be understood that one could just as well reverse the high coercivity layers in R2 and R3 to achieve the same effect.

[0123] If one instead reverses the high coercivity layers in R1 and R3, this merely reverses the polarity of the output, i.e., the switch conducts with full output for one input polarity. The same is true for switching the high coercivity layers of R2 and R4.

[0124] If one reverses the high coercivity layer in only one of the films, the result is a switch with half the output. Reversing the magnetization of the low coercivity layers with an input current does not shut the switch off. Reversing the high coercivity layers in three of the GMR elements (e.g., R1, R2, and R3) also reduces the output by half.

[0125] In summary, the two schemes that turn switch 1200 off for both input polarities are either to reverse the magnetization of the high coercivity layers in R1 and R4 or to reverse the magnetization of the high coercivity layers in R2 and R3.

[0126] Transpinnor switch 1200 has an input conductor 1202 coupled to only two thin-film elements, i.e., R1 and R3. Transpinnor switches designed according to various other embodiments of the invention may have an input conductor over all four thin-film elements in the bridge. Such a switch 1300 is shown in FIG. 13a. Input conductor 1302 is coupled to R1-R4 and switch 1300 can be configured to pass input currents of both polarities. Transpinnor switch 1300 can be switched off by reversing the magnetization in high-coercivity layer 1304 of either R1 and R4, or R2 and R3. It should be noted that the switch conductor by which this reversing is accomplished is not shown in FIG. 13a for the sake of simplicity and because of the fact that it can be coupled to either combination of R1 and R4 or R2 and R3.

[0127] Transpinnor switch 1300 is shown in FIG. 13b with switch conductor 1306 inductively coupled to R1 and R4. This configuration passes both polarities of input current if the high-coercivity layers 1304 of all four thin-filn elements are magnetized in the same direction, and blocks all polarities of input current when the magnetization of high-coercivity layers 1304 is reversed in R1 and R4. A similar embodiment (not shown) has the switch conductor associated with R2 and R3.

[0128] It should be noted that, according to various embodiments, the transpinnor switch of the present invention may be used for either digital or analog applications. According to an embodiment in which a transpinnor switch is employed to transmit analog signals, the switching field generated by the switching current is perpendicular to the easy-axis of the GMR elements rather than parallel. When the switch is enabled, the switching bias field is raised above the anisotropy field of the lower-coercivity layer. This causes the transpinnor to operate like a linear, nearly lossless transformer. When the bias field is turned off, there is no output unless the signal is large enough to exceed the coercivity of the lower-coercivity layer. In this way, analog signals may also be routed point-to-point according to the invention.

[0129] According to various embodiments of the invention, the transpinnor switch of the present invention may be configured to output an amplified version of the input. That is, using the techniques described above, transpinnor switches may be configured to provide a wide range of amplification factors including negative amplification factors, i.e., a transpinnor switch may be configured as an inverter. This amplification capability can be important for applications in which it is desirable to cascade transpinnor switches (see FIG. 14b).

[0130]FIG. 14a shows a circuit symbol representing a transpinnor switch 1400 having an input conductor 1402, a switch conductor 1404, and an output 1406. FIG. 14b shows three such transpinnor switches 1400 sharing the same input conductor 1402 but with three separate outputs 1406, each of which can be turned on or off with the corresponding switch conductor 1404.

[0131] The ability to cascade transpinnor switches is also advantageous for creating switching matrices such as the 2×2 switching matrix 1500 shown in FIG. 15. More specific implementations of some exemplary switching matrices will now be described with reference to FIGS. 16a and 16 b. It will be understood that the switching matrices shown may be used in a wide variety of context to effect the interconnection of signal paths for any of a variety of purposes. It will also be understood that these basic switching matrices may be expanded beyond the sizes shown to selectively interconnect any of a first plurality of m signal lines with any of a second plurality of n signal lines, i.e., an m×n switching matrix. The relevance of this will become apparent with reference to applications of the transpinnor switch of the present invention described below.

[0132]FIG. 16a shows a specific implementation of a 2×2 switching matrix 1600 corresponding to the simplified representation of FIG. 15. Each of the inputs can be switched into any of the outputs. Thus it is possible, for example, to connect input 1 to output 1, and input 2 to output 2, or, input 1 to output 2 and input 2 to output 1. One can have as many inputs to a transpinnor as desired, because an input line may be implemented as a stripline deposited above a GMR film and insulated therefrom, and one can have many such lines, one above another. FIG. 16b shows a specific implementation of a 3×3 switching matrix 1650, in which any input can be connected to any output.

[0133] Programmable logic devices (PLDs) are a class of circuits widely used in LSI and VLSI design to implement two-level, sum-of-products Boolean functions. PLDs include programmable array logic (PALs), field programmable gate arrays (FPGAs), programmable logic arrays (PLAs), and read only memories (ROMs). One of the advantages of PLDs is their highly regular layout structure. That is, a typical PLD includes an AND plane followed by an OR plane. The logic function performed by the device is determined by the presence or absence of contacts or connections at row and column intersections in a single conducting layer.

[0134] FPGAs are the most flexible of the PLDs in that they can be reconfigured multiple times. In conventional semiconductor technology, FPGA implementations typically choose between two very different types of memories to control their switches, one which ensures nonvolatility, a one which ensures speed. A block diagram of a generic FPGA architecture is shown in FIG. 17. FPGA 1700 has n inputs, k product terms (AND array 1702), m sum terms (OR array 1704), m output, and (2nk+mk+2m) switches. In conventional semiconductor technology, the switches are typically implemented with MOSFETs in conjunction with either EEPROMs or SRAM which may be reloaded with different data to reconfigure the FPGA. A significant drawback with the MOSFET/SRAM combination is that if power is lost, the SRAM must be reloaded for the FPGA to function properly. On the other hand, although EEPROMs avoid this issue because they are nonvolatile, they are an order of magnitude slower than SRAM, limiting the reprogramming speed of an EEPROM-based FPGA accordingly.

[0135] Therefore, according to the present invention, an FPGA architecture is provided which employs any of the various embodiments of the transpinnor switches described herein (e.g., transpinnor switch 1200) as the basis for the switch matrices with which the product and sum terms of the FPGA may be interconnected. The nonvolatile nature of the state of these transpinnor switches, and the speed with which they may be accessed and switched results in a solution which has the best of both previous options without the attendant disadvantages. According to some of these embodiments, the product terms and sum terms of the FPGA (e.g., arrays 1702 and 1704) are implemented using transpinnor logic gates including, for example, those described above with reference to FIGS. 8-10.

[0136] According to various embodiments, an FPGA architecture designed according to the invention may conform to the conventional paradigm in which switches are controlled by associated memory elements. According to some of these embodiments, the memory elements may be implemented using any of the GMR-based memory cells described in U.S. Pat. No. 5,587,943 for NONVOLATILE MAGNETORESISTIVE MEMORY WITH FULLY CLOSED FLUX OPERATION issued Dec. 24, 1996, and International Publication No. WO 02/05268 A2 entitled ALL METAL GLANT MAGNETORESISTIVE MEMORY published Jan. 17, 2002, the entire disclosures of both of which are incorporated herein by reference for all purposes. Such memory elements will be referred to herein generically as SpinRAM elements.

[0137] According to other embodiments, the nonvolatile nature of the transpinnor switch of the present invention obviates the need for controlling the switches with associated memory elements. That is, because the state of the switches designed according to the invention is nonvolatile, the switches themselves may be directly programmed as opposed to indirectly programming them via the associated memory elements.

[0138] According to still other embodiments, one or more FPGAs designed according to the present invention is included as part of a larger, field programmable system-on-a-chip (FPSOC). One such generalized embodiment is shown in FIG. 18. According to various embodiments, any or all of the system components of FPSOC 1800 may be based on all-metal GMR electronics. For example, FPGA 1802 may be implemented as described above using transpinnor switch matrices and transpinnor logic gates. Some or all of the mixed signal components of field programmable analog array 1804 (e.g., differential amplifiers, sample-and-hold circuits, etc.) may be implemented using transpinnor-based circuits. In addition, memory array 1806 may be implemented as a SpinRAM array. Arithmetic logic unit 1808, multiply/accumulate unit 1810, and general purpose I/O 1812 may all be implemented using transpinnor logic gates.

[0139] A common method of reconfiguring a FPSOC in conventional semiconductor technology employs logic gates for routing configuration signals. According to one embodiment of the invention, such a method can also be implemented using the transpinnor logic gates described herein. According to other embodiments, advantage is taken of the unique aspects of transpinnor technology to provide a simpler approach using transpinnor switches to reconfigure such FPSOCs.

[0140] Generally speaking, PLDs, FPGAs, and FPSOCs designed according to the invention may implement any of the wide variety of functions and be employed in any of the wide variety of applications and environments as any of their conventional counterparts. In addition, for embodiments in which all of the circuit components, functional blocks, and subsystems are based on the all-metal GMR technology described herein, several advantages over conventional semiconductor or hybrid implementations will be enjoyed. That is, such all-metal circuits and systems are intrinsically radiation-hard. From a manufacturing standpoint, fewer processing steps, lower processing temperatures, and fewer masks, make such all-metal implementations logistically and economically superior. Single transpinnor implementations of conventionally more complicated circuits, e.g., logic gates, differential amplifiers, sample-and-hold circuits, comparators, etc., and the closed-flux nature of some memory elements facilitate increased density as well more reliable performance.

[0141] In some implementations of systems incorporating all-metal electronics, transmission line effects need to be taken into account. That is, for example, in systems in which signals are transmitted at high speeds or over long distances, it is important that the terminating impedances at the ends of a transmission line match its characteristic impedance in order to transmit the signals reliably and efficiently. This is particularly true, for example, in the complex architectures that can be implemented using GMR technology. More specifically, the transmission of information between multiple GMR chips in such systems will often need to deal with such transmission line effects.

[0142] A simple model for the transmission of signals over a line is given by its characteristic impedance, i.e., the instantaneous equivalent resistance that the transmitter sees at the source end when a signal voltage is applied, normally denoted Z₀. It is a function of the separation distance, separating dielectric material, and geometry of the two conductors. In a uniform transmission line the characteristic impedance looking back from any point along the line is the same. A coaxial cable typically has the most uniform geometry and impedance, but even a twisted pair is sufficiently uniform to be very effective as a transmission medium. Typical characteristic impedances for lines used to transmit digital information are 50 to 200 ohms.

[0143] When both the source and destination are terminated with resistors that have the same value as the characteristic impedance, optimum power is transmitted and there are no signal reflections. Having no reflections is important to prevent successive signals from interfering at a destination node when the travel time over the transmission line gets to be greater than the cycle time of the signals. For example, if a signal cycle time is 1 ns (about 1 foot of travel) and the transmission line is more than 10 ft there can be as many as 10 signals traveling on the transmission line between the transmitter and receiver. Any excessive reflections will interfere with the reception of the real signals.

[0144] When a signal waveform is applied at the source node of a transmission line, it takes a predictable amount of time to travel down the line, i.e., the delay due to an electromagnetic wave traveling at the speed of light in the dielectric medium separating the two transmission conductors. A reasonable working approximation is to take the speed to be 1 foot/ns. The actual delay depends on the material between the conductors (e.g., the speed of light in conventional coaxial cable or twisted pair conductors is roughly 0.6 c, or about 1.5 ft/ns). For normal circuitry this is not an issue. The fact that the delays can be predicted allows interconnects to be designed with predictable delays and compensating circuits if the delays are significant. For short connections, transmission-line delays may be ignored as long as design rules governing these delays are in place. When multiple transmission lines carry parallel signals that need to be synchronized, skew in the arrival of the signals at their destination may have to be taken into account. Many solutions are available (including the lengthening of the shorter transmission lines) to ensure that there is minimal skew. When the clock speed of digital logic gets close to the transmission speed of signals between the logic components, interconnects may have to be treated as transmission lines even though they may only appear to be short printed-circuit traces or internal interconnects on a chip.

[0145] Reflections of signals are the main source of problems in short-distance transmission-line systems. When a transmission line is properly terminated at both ends with the same resistance as the characteristic impedance, a waveform (in this case a pulse) is repeated, with the proper delay, at any point in the transmission line. There is no reflection of the waveform. Two limiting cases of the terminating node are of interest, open and shorted. The waveform for a circuit open at the termination and properly terminated at the source is shown in FIG. 19A.

[0146] Until the round-trip delay is completed, the source sees the equivalent of a terminated transmission line. Since the energy arriving at the termination is not absorbed, it is reflected as a negative current that will cause the voltage at the source to overshoot. As successive reflections appear at the source, the waveform gradually dampens out to its steady state and the current flowing in the transmission line goes to 0. If the transmission line is very short compared to the signal length, it will become equivalent to a capacitor. When a fast pulse is applied, the initial response is due to the capacitor charging and, when charged, the transients die down. Therefore a short unterminated transmission line can be replaced with an equivalent capacitance for purposes of both modeling and design.

[0147] The second limiting case of interest is when the terminating node of a transmission line is shorted. The waveform for a pulse applied at the source node is shown in FIG. 19B. Each step represents a round-trip delay in the transmission line. Initially the line appears to the source as a normal terminated line and the normal current is sent. When the current arrives at the destination, it is not absorbed and is therefore reflected negatively. When arriving back at the source, the equivalent voltage is reduced. The bouncing back and forth continues until there is no voltage across the source terminals and the maximum current flows through both conductors of the transmission line.

[0148] The use of semiconductors as drivers and receivers for transmission lines has been developed over many years, and many commercial products are available. For example, standards-based system interconnects such as the RS485 are used heavily for interconnecting systems. There are families of RS485 transceivers that make implementing an interface to an RS485 bus very easy and commonplace. The advantage of such standards-based transmission-line systems is that they are vendor and technology independent.

[0149] More and more interconnects are gravitating toward serial transmission of information as a replacement for parallel lines at all levels of systems. The Universal Serial Bus (USB) is an example. The transmission line is only point-to-point, and the terminations are well matched to get very good transmission performance. Internal buses like the SCSI bus also depend on transmission-line characteristics of interconnects and well-matched terminations. In this case the drivers and receivers are built into the host, and peripheral subsystems are based on well-defined standards.

[0150] In SERDES (serializer/deserializer) schemes, an array of parallel inputs is sampled by a serializer and then sent at a very high speed to the other end of the transmission line, where a deserializer builds the parallel version and gates it onto the output as parallel signals. Many of these SERDES operate at a gigahertz, some are now running above 3 GHz, with plans to reach 10 GHz. The main advantage of this approach is that many parallel interconnects can be eliminated with a single transmission-line replacement. In systems incorporating all-metal electronics such as those described above, similar techniques may be employed.

[0151] As will become clear, all-metal solutions corresponding to such transceiver types (as well as many others) may be provided according to various embodiments of the invention. More specifically and according to such various embodiments of the invention, appropriately configured transpinnors are provided as drivers and receivers for transmission lines.

[0152] According to various embodiments, a variety of transpinnor characteristics may contribute to their suitability for implementation as transceiver components. For example, as described above, the output terminals of transpinnors are electrically isolated from their inputs to the extent of the strength of the dielectric between them. This helps inhibit unintended subsystem interactions such as ground loops and noise. In addition, the outputs of a transpinnor are the bridge terminals of a four GMR element bridge structure. As a result, as the transpinnor is switched, the resistance seen at the output terminals remains substantially constant. This assumes that the GMR elements are well balanced; a situation that should be easy to achieve since all four elements are adjacent and should have the same GMR response curve.

[0153] As discussed above, to prevent reflections and noise the source impedance of a transmission line driver should be the same as the characteristic impedance of the transmission line. Transpinnors can be configured by adjusting their length and width so that the output terminals present any desired impedance to the transmission line. Moreover, when the transmitting transpinnor switches polarity the output side power supply and ground terminals of the transpinnor are essentially noise-free. The reason for this is that the switching of the individual GMR elements in the transpinnor is done symmetrically and does not result in a significant impedance change from the perspective of the power supply. As result, little or no ground current perturbations are experienced by the power supply. This is in contrast to CMOS line drivers where rail-to-rail switching may induce large transient spikes producing power supply noise problems for the rest of the circuit.

[0154] When a transpinnor is configured as a receiver according to specific embodiments of the invention, its input is nearly equivalent to a short circuit. This allows multiple receivers to be deployed on the same transmission line thereby supporting large fan-outs. Transpinnor receivers can also be used as taps on a transmission line since they absorb little or no energy and therefore do not greatly affect the transmission line characteristic. It will be understood, that the low impedance input of transpinnor receivers means that the transmission line should be terminated by an impedance corresponding, for example, to the transmission line characteristic impedance.

[0155] Transpinnors may also be configured to have noise-tolerant receiver input characteristics. That is, because of the nature of these GMR devices, they can be configured as receivers such that small signals at the transpinnor input (e.g., from noise or transmission line reflections) can be tolerated by properly biasing and conditioning the GMR elements such that a strong enough signal must be received for the input to switch to the opposite polarity.

[0156] Another advantage of transpinnor-based transceiver components is that the GMR elements for both transmitters and receivers can be designed so that they have memory. In the case of receivers, this capability makes it possible to receive and remember very short transmitted pulses. This can be important, for example, in transmission line systems with very low power, or in control systems in which the amount of information to be transmitted is low and relatively infrequent but where the transmission power must nevertheless be minimized.

[0157] On the transmitter side, this allows the transmitter to be set by input pulses (e.g., the driving logic is pulse based), and allows the output side to be pulsed whenever convenient. This could be useful, for example, where it is desirable that the output pulse which produces the transmitted signal is independent of the timing of the corresponding input pulse, i.e., the transmitter acts as a holding register.

[0158] For transpinnor-circuit to transpinnor-circuit transmissions a variety of contexts will be encountered, e.g., traces connecting driving transpinnors with receiving transpinnors on a single chip, signal lines between chips routed via printed circuit boards (PCBs), and signal lines between PCBs or subsystems. The transmitting medium can be any of a wide variety of mediums including, for example, coaxial cable, twisted pair, or PCB stripline conductors.

[0159] According to a particular embodiment and as described above, a transpinnor driver is configured as a set of four GMR elements interconnected in a Wheatstone bridge. The GMR elements can be adjusted at design time (or even subsequently configured) such that the driver presents a source impedance to a transmission line which is at or near the characteristic impedance of the line, i.e., the transpinnor can be used to directly drive the transmission line. A circuit diagram for an exemplary transpinnor driver is shown in FIG. 20.

[0160] Each of the GMR elements in transpinnor driver 2000 has an impedance value Z₀*(1−gmr/2), where Z₀ is the characteristic impedance of the transmission line, and gmr is the fractional GMR value of the GMR elements. Driver 2000 may operate in a variety of manners depending on the particular application. According to one implementation, power is always applied to driver 2000 at terminals CP and CN, and the output signal is controlled by the input signal at terminals INP, INN. With a logic 0 input, e.g., 0 volts, driver 2000 is configured so that there is no voltage at the output terminals OUTP and OUTN. When the input is changed to a logic 1, e.g., 3-5 volts, the magnetizations of the GMR elements are flipped, the bridge becomes unbalanced, and a positive voltage appears at the OUTP, OUTN terminals. Thus, information is transmitted as positive pulses on the transmission line. It will be understood that the design can also be altered slightly to allow negative pulses to represent information.

[0161] According to another implementation, power is again always applied to transpinnor driver 2000 and either a negative or a positive output current flows depending on the state of the input. This mode allows the receiving nodes to detect if the driver is operational, i.e., the absence of current can indicate a special condition or a circuit fault.

[0162] According to yet another implementation, the power to the transpinnor at terminals CP and CN is only applied when a pulse is sent out. The input signal or the state of the GMR elements determines whether the transmitted pulse is negative or positive. This implementation is characterized by some significant advantages. By pulsing the output, energy is conserved. In addition, the fact that the output pulse is going either negative or positive both allows the information to be transmitted and allows the signal to be self-clocking. This approach does require a receiver which is able to capture and discriminate the incoming pulse. Therefore, according to another specific embodiment of the invention, a transpinnor receiver is provided which has such a capability.

[0163] Because the input of a particular implementation of a transpinnor is a stripline with essentially zero resistance, such transpinnors are natural transmission line receivers. That is, the input of such a transpinnor can be used as a tap on a transmission line with minimal theoretical interruption to the signal on the line.

[0164]FIG. 21 shows a possible implementation with a number of transpinnor receivers 2100 (T2-Tn) connected to a transmission line 2102. Transmission line driver 2104 (T1) on the left sends a signal into transmission line 2102 that is eventually terminated with the resistor Rt. For each of receivers 2100, a portion of transmission line 2102 is cut and the stripline input of the transpinnor receiver is spliced in. Because the resistance of the stripline is very low the disruption in the transmission line is minimal to a first-order approximation.

[0165] According to various embodiments, the combination of the signal current in the transmission line and the sensitivity of the transpinnor input are sufficient to cause the soft layer of the GMR elements of the receiving transpinnor to switch. When this occurs, the output senses the change directly. According to specific embodiments, the receiving transpinnor can also be designed to have memory to store the input pulse and polarity. According to such embodiments, the transpinnor can be powered at a later time to read the state that was captured.

[0166] According to various embodiments of the present invention, both static and pulsed transceiver circuits may be constructed using transpinnor-based transmitters and receivers such as those illustrated in FIGS. 20 and 21. A specific implementation of a static transpinnor transmitter designed according to the invention can be characterized in the following manner. The GMR element resistances of such a transmitter are adjusted so that the output impedance (on the bridge output pins) is substantially the same as the characteristic impedance of the transmission line.

[0167] In addition, the transmitter is designed to provide enough current drive to be able to switch the receiving transpinnors. For example, if Imin is the minimum receiving current, the driving voltage at the transmitter side should be at least Z₀*Imin volts. Since this is the voltage produced at the transmitter bridge terminals, the actual driving voltage must be at least (Z₀*Imin)*2/gmr where gmr is the fractional representation of the GMR value. For example, if the receiver switches fully with 2 ma of current, the driving voltage at the bridge terminals must be (50*0.002)=0.1 volts. However, the terminal equivalent driving voltage is 0.2 volts since both the transpinnor equivalent resistance and the transmission line equivalent resistances divide the voltage. For a transpinnor with a GMR of 5%, gmr=0.05, and the actual transmitter driving voltage for this example is then (0.2/gmr)=4.0 volts. Note that the better the GMR, the lower the driving voltage required. Note also that the transmission power efficiency is proportional to the GMR of the transpinnor.

[0168] A specific implementation of a static transpinnor receiver designed according to the invention can be characterized in the following manner. At a specified threshold current level such a transpinnor receiver switches output states. This threshold discriminates between logic levels on the transmission lines. FIG. 22 illustrates how a transpinnor element with hysteresis may be used to achieve switching threshold 2202 and noise tolerance 2204. As can be seen from the figure, the non-linearity of the GMR elements is used to tolerate noise on the transmission line. According to alternative embodiments, it is also possible to use transpinnors without hysteresis but with a pinning layer to offset the curve to the switching level.

[0169] According to various embodiments of the invention, a pulsed transpinnor transceiver may be constructed with GMR elements that have memory. According to one such embodiment, the transpinnor elements of such a transceiver are designed with the characteristic curve of FIG. 23. This curve applies to both the transpinnor used as a transmitter and also as a receiver. According to a specific embodiment, the characteristics of the receiver are more carefully adjusted so that the switching threshold is as crisp as possible and the noise tolerance is at an acceptable level.

[0170]FIG. 24 shows an exemplary configuration of a transpinnor 2400 which may be used as either a receiver or a transmitter in a pulsed transceiver embodiment. When transpinnor 2400 is not energized, it may be in either a high or low state. The states in the 4 GMR elements are balanced so that elements 1 and 3 are in the same state and elements 2 and 4 are in the opposite state. Signal lines TXP and TXN represent positive and negative data input controls. Signal lines CLRP and CLRN are control inputs used to reset the state of the transpinnor in between receiving data. Signal lines VDP and VDN are output activation driver voltages.

[0171] Where transpinnor 2400 is employed as a transmitter, VDP and VDN are pulsed to send a signal onto the transmission line. Where transpinnor 2400 is employed as a receiver, these two lines are activated to read the state of the transpinnor. Signal lines RXP and RXN are the output terminals. For the transmitter, they are connected to the transmission line and for the receiver, they are connected to other logic circuitry (which may comprise transpinnor-based circuitry) that processes the input data. Signal lines HCLRP and HCLRN are additional input controls which may be provided for controlling the proper biasing of the hard (Cobalt) GMR layer.

[0172]FIG. 25 represents an exemplary sequence of bits transmitted from a transmitter to a receiver according to a particular embodiment of the invention. The signal line names and reference numbers from FIG. 24 are employed. The TXD trace shows data being clocked into the transmitter. These data are clocked in after the TXCLR trace has reset the receiver input. As shown, the TXD trace has the bit stream <1, 0, 1, 1> being entered. The pulses for 1's are high enough to change the receiver state to 1 and the 0 data bit pulse leaves it in state 0.

[0173] The TXR13 trace represents the state of the resistance in GMR elements 1 and 3. After a 1 is entered the resistance is at the high level corresponding to the high state. The resistances for GMR elements 2 and 4 are opposite polarities. The TXCLR trace represents the transmitter clear pulse. This pulse is sent after each bit is transmitted. It puts the transmitter into low state.

[0174] The TXP trace is the transmit pulse applied across the VDP and VDN transpinnor terminals. This pulse activates the output of the transmitter and produces either a positive or negative pulse on the transmission line depending on the state of the transmitter. Note that the output level versus input level is proportional to the GMR of the device. The outgoing pulse must be strong enough and long enough to switch the receiver transpinnors.

[0175] The TXLINE trace is the voltage and current waveform seen on the transmission line itself. It is typically delayed and degraded from the input pulse due to capacitances and irregularities in the transmission line. As the pulse travels down the transmission line, it must be strong enough to switch the receiving transpinnors. Note that a positive pulse is sent with a data bit of 1 and a negative pulse with a data bit of 0. It should be noted that this can be used in a more refined system to make the transmissions self-clocking.

[0176] On the receiver side, a clear pulse (the RXCLR trace) is used in between data bits to clear the receiver to a 0 state. This signal can be derived form the transmission line directly. Alternatively, it can be derived from a separate clock source that synchronizes transmits and receives. The RXR13 trace represents the state of the GMR resistance in the receiving transpinnor's GMR elements 1 and 3. Note that after a receipt of a positive pulse, the resistance becomes high and after a negative pulse, it remains low. The RXD trace indicates that receiver data are available. That is, this signal shows the periods in which the receiving transpinnor can be sampled for the received data.

[0177] For a full GMR implemented solution, all of the logic for buffering and sequencing of the transmit and receive portions of the system may also be implemented as GMR elements. FIG. 26 shows the components for such an implementation. According to various implementations, different sequence logic implementations are provided for a wide variety of classes of transmissions. For example, a simple interface might just mimic a standard RS232 interface implemented in GMR logic. A more advanced application may be the messaging part of a TCP/IP subsystem. Any of the myriad other possibilities will be apparent to one of skill in the art.

[0178] While the invention has been particularly shown and described with reference to specific embodiments thereof, it will be understood by those skilled in the art that changes in the form and details of the disclosed embodiments may be made without departing from the spirit or scope of the invention. For example, embodiments have been described above with reference to conventional interface types which the transceiver components of the present invention may be configured to emulate. It should be noted that any references to such interfaces are merely exemplary and should not in any way be construed to limit the scope of the invention.

[0179] It should also be noted, for example, that the basic transpinnor topology of the present invention is not limited to any particular magnetic materials, bridge configuration, number of thin-film periods, etc. That is, any materials, configurations, periodicity, etc., of thin-film structures which are appropriate to enable the functions described herein are contemplated to be within the scope of the invention. It is also important to note that the transceiver component configurations described herein are merely exemplary and that many functionally equivalent configurations are within the scope of the invention.

[0180] Finally, although various advantages, aspects, and objects of the present invention have been discussed herein with reference to various embodiments, it will be understood that the scope of the invention should not be limited by reference to such advantages, aspects, and objects. Rather, the scope of the invention should be determined with reference to the appended claims. 

What is claimed is:
 1. A driver for a transmission line, comprising a network of thin-film elements exhibiting giant magnetoresistance, and an input conductor inductively coupled to at least one of the thin-film elements, wherein the driver is operable to generate an output signal which is a function of a resistive imbalance among the thin-film elements and which is proportional to a power current in the network of thin-film elements, and wherein at least one dimension of each thin-film element is configured with reference to a characteristic impedance of the transmission line.
 2. The driver of claim 1 wherein the at least one dimension of each thin-film element comprises at least one of length and width.
 3. The driver of claim 1 wherein the at least one dimension of each thin-film element is configured such that the driver has a source impedance which substantially matches the characteristic impedance of the transmission line.
 4. The driver of claim 3 wherein magnetizations of layers in the thin-film elements are configured such that the source impedance of the driver more closely matches the characteristic impedance of the transmission line.
 5. The driver of claim 4 further comprising a biasing conductor inductively coupled to selected ones of the thin-film elements to effect configuration of the magnetizations.
 6. The driver of claim 1 wherein the at least one dimension of each thin-film element is configured such that each thin-film element has a corresponding impedance given by Z₀*(1—gmr/2), where Z₀ is the characteristic impedance of the transmission line, and gmr is a fractional giant magnetoresistive value of the corresponding thin-film element.
 7. The driver of claim 1 wherein the thin-film elements are operable to collectively store a signal state for transmission on the transmission line.
 8. The driver of claim 7 wherein the driver is operable to transmit the state in response to an enabling pulse.
 9. The driver of claim 8 wherein the enabling pulse corresponds to the power current.
 10. The driver of claim 1 wherein the driver is configured to transmit the output signal in response to an enabling pulse.
 11. The driver of claim 10 wherein the enabling pulse corresponds to the power current.
 12. The driver of claim 1 wherein the driver is configured to transmit the output signal in response to an input current in the input conductor.
 13. The driver of claim 12 wherein the driver is configured such that the output signal is zero when the input current is zero.
 14. The driver of claim 12 wherein the driver is configured such that the output signal comprises either a positive current or a negative current depending on the input current.
 15. The driver of claim 1 further comprising a reset conductor inductively coupled to the at least one of the thin-film elements, the reset conductor being operable to reset a state associated with the at least one of the thin-film elements.
 16. The driver of claim 1 wherein the network of thin-film elements comprises four thin-film elements configured in a Wheatstone bridge, and wherein the input conductor is inductively coupled to all of the thin-film elements.
 17. The driver of claim 1 wherein each thin-film element comprises a multilayer structure having a plurality of periods of layers.
 18. The driver of claim 17 wherein each period of layers comprises a first magnetic layer characterized by a first coercivity, a second magnetic layer characterized by a second coercivity, and a nonmagnetic conducting layer interposed between the first and second magnetic layers.
 19. The driver of claim 1 wherein the network of thin-film elements forms a closed flux structure.
 20. The driver of claim 1 wherein the network of thin-film elements forms an open flux structure.
 21. The driver of claim 1 wherein the resistive imbalance among the thin-film elements corresponds to an input current in the input conductor.
 22. The driver of claim 21 wherein the power current is continuously applied to the network of thin-film elements.
 23. The driver of claim 1 wherein the resistive imbalance among the thin-film elements corresponds to a state stored in the network of thin-film elements.
 24. The driver of claim 23 wherein the power current is applied as discrete pulses to the network of thin-film elements.
 25. An electronic system comprising a plurality of circuits, and a transmission line between selected ones of the circuits, a first one of the selected circuits comprising a driver coupled to the transmission line, the driver comprising a network of thin-film elements exhibiting giant magnetoresistance, and an input conductor inductively coupled to at least one of the thin-film elements, wherein the driver is operable to generate an output signal which is a function of a resistive imbalance among the thin-film elements and which is proportional to a power current in the network of thin-film elements, and wherein at least one dimension of each thin-film element is configured with reference to a characteristic impedance of the transmission line.
 26. A receiver for a transmission line, comprising a network of thin-film elements exhibiting giant magnetoresistance, and an input conductor inductively coupled to at least one of the thin-film elements, wherein the receiver is operable to generate an output signal which is a function of a resistive imbalance among the thin-film elements and which is proportional to a power current in the network of thin-film elements, the receiver further comprising a termination impedance in series with the input conductor, a value of the termination impedance relating to a characteristic impedance of the transmission line.
 27. The receiver of claim 26 wherein the value of the termination impedance is such that the receiver presents a load impedance to the transmission line which substantially matches the characteristic impedance of the transmission line.
 28. The receiver of claim 26 further comprising a biasing conductor inductively coupled to selected ones of the thin-film elements to effect configuration of magnetizations of layers in the selected thin-film elements.
 29. The receiver of claim 26 wherein the thin-film elements are operable to collectively store a signal state received via the transmission line.
 30. The receiver of claim 29 wherein the receiver is operable to transmit the state in response to an enabling pulse.
 31. The receiver of claim 30 wherein the enabling pulse corresponds to the power current.
 32. The receiver of claim 26 wherein the receiver is configured to transmit the output signal in response to an enabling pulse.
 33. The receiver of claim 32 wherein the enabling pulse corresponds to the power current.
 34. The receiver of claim 26 wherein the receiver is configured to transmit the output signal in response to an input current in the input conductor.
 35. The receiver of claim 34 wherein the receiver is configured such that the output signal is zero when the input current is zero.
 36. The receiver of claim 34 wherein the receiver is configured such that the output signal comprises either a positive current or a negative current depending on the input current.
 37. The receiver of claim 26 further comprising a reset conductor inductively coupled to the at least one of the thin-film elements, the reset conductor being operable to reset a state associated with the at least one of the thin-film elements.
 38. The receiver of claim 26 wherein the network of thin-film elements comprises four thin-film elements configured in a Wheatstone bridge, and wherein the input conductor is inductively coupled to all of the thin-film elements.
 39. The receiver of claim 26 wherein each thin-film element comprises a multilayer structure having a plurality of periods of layers.
 40. The receiver of claim 39 wherein each period of layers comprises a first magnetic layer characterized by a first coercivity, a second magnetic layer characterized by a second coercivity, and a nonmagnetic conducting layer interposed between the first and second magnetic layers.
 41. The receiver of claim 26 wherein the network of thin-film elements forms a closed flux structure.
 42. The receiver of claim 26 wherein the network of thin-film elements forms an open flux structure.
 43. The receiver of claim 26 wherein the network of thin-film elements is configured to switch between logic states in response to an input current in the input conductor.
 44. The receiver of claim 43 wherein the network of thin-film elements is configured to switch between the logic states with hysteresis.
 45. The receiver of claim 43 wherein the network of thin-film elements is configured to switch between the logic states without hysteresis, selected thin-film elements comprising a pinning layer to offset a switching threshold associated with switching between the logic states.
 46. The receiver of claim 1 wherein the resistive imbalance among the thin-film elements corresponds to an input current in the input conductor.
 47. The receiver of claim 21 wherein the power current is continuously applied to the network of thin-film elements.
 48. The receiver of claim 1 wherein the resistive imbalance among the thin-film elements corresponds to a state stored in the network of thin-film elements.
 49. The receiver of claim 48 wherein the power current is applied as discrete pulses to the network of thin-film elements.
 50. An electronic system comprising a plurality of circuits, and a transmission line between selected ones of the circuits, a first one of the selected circuits comprising a receiver coupled to the transmission line, the receiver comprising a network of thin-film elements exhibiting giant magnetoresistance, and an input conductor inductively coupled to at least one of the thin-film elements, wherein the receiver is operable to generate an output signal which is a function of a resistive imbalance among the thin-film elements and which is proportional to a power current in the network of thin-film elements, the receiver further comprising a termination impedance in series with the input conductor, a value of the termination impedance relating to a characteristic impedance of the transmission line.
 51. A transceiver for a transmission line, comprising: a driver for the transmission line, comprising a first network of thin-film elements exhibiting giant magnetoresistance, and a first input conductor inductively coupled to at least one of the thin-film elements in the first network, wherein the driver is operable to generate a first output signal which is a function of a resistive imbalance among the thin-film elements in the first network and which is proportional to a first power current in the first network of thin-film elements, and wherein at least one dimension of each thin-film element in the first network is configured with reference to a characteristic impedance of the transmission line; and a receiver for the transmission line, comprising a second network of thin-film elements exhibiting giant magnetoresistance, and a second input conductor inductively coupled to at least one of the thin-film elements in the second network, wherein the receiver is operable to generate a second output signal which is a function of a resistive imbalance among the thin-film elements in the second network and which is proportional to a second power current in the second network of thin-film elements, the receiver further comprising a termination impedance in series with the second input conductor, a value of the termination impedance relating to the characteristic impedance of the transmission line.
 52. A transceiver for a transmission line, comprising: a driver for the transmission line, comprising a first network of thin-film elements exhibiting giant magnetoresistance, and a first input conductor inductively coupled to at least one of the thin-film elements in the first network, wherein at least one dimension of each thin-film element in the first network is configured with reference to a characteristic impedance of the transmission line; and a receiver for the transmission line, comprising a second network of thin-film elements exhibiting giant magnetoresistance, and a second input conductor inductively coupled to at least one of the thin-film elements in the second network, the receiver further comprising a termination impedance in series with the second input conductor, a value of the termination impedance relating to the characteristic impedance of the transmission line. 